Slurries of coal/water mixtures comprising finely ground coal dispersed in water have been found to be useful as fuels in place of petroleum in steam boilers and thermal engines, such as diesel engines. (Fifth International Symposium on Coal Slurry Combustion and Technology Center, Apr. 25-27, 1983; note also "Coal: Diesel Fuel of the Future?" by John Dunlay, Power, Vol. 125, No. 3, March 1981, pp. 43-44).
According to U.S. Pat. No. 4,282,006, grinding requirement and additives needed to produce stable slurries of acceptable viscosities and coal contents are well established. In the patent, the background of the prior art is discussed in quite some detail and, as to the relevant prior art discussed therein, the disclosure of said patent is incorporated herein by reference.
In some applications, such as in diesel engines, it may be desirable, for example, to switch from one type of fuel (e.g., diesel oil) to another, such as coal/water mixtures, which is referred to herein as CWM. Because petroleum oils, such as No. 2 diesel fuel, are more easily ignited than coal slurries, the diesel engines are best started with petroleum oil followed by switching over to CWM fuel soon after the engine is started and finally back to oil shortly before stopping the engine. During the switch-over periods, the two types of fuels must be compatible during transitional mixing since the flow of both fuels is necessarily through the same pipelines feeding the combustion chamber of the engine.
The usual formulations of the coal/water mixture are not compatible with oil. When mixed with No. 2 diesel fuel or with No. 6 fuel oil, these CWM fuels form exceedingly high-viscosity masses of agglomerated coal. Viscosities of 400,000 to 640,000 centipoise have been measured for these masses. Such masses can clog lines and orifices. The reason for this agglomeration is that coal surfaces are normally hydrophobic and oleophillic. Any oil added to the system displaces water from the coal surfaces and binds the coal particles tightly together. The dispersants ordinarily added to the coal/water mixtures (such as Diamond-Shamrock A-23) are active enough to overcome the hydrophobic nature of the coal sufficiently so that low-viscosity coal-water mixtures form but these dispersants are not powerful enough to overcome the oleophillic nature of the coal.
It would be desirable to provide a coal/water mixture that can be mixed with petroleum, such as fuel oil, without causing the powdered coal to agglomerate and raise the viscosity of the mixture to unacceptable levels.